This invention relates to a power supply apparatus for supplying a device with electric energy and, comprising at least one test input for receiving a test signal which is dependent on a variable which itself is dependent on the power applied to the device. The test input is connected to a first input of a comparator circuit, a second input of which is connected to a generator which is adapted to generate a reference signal which is a measure of a desired value of said variable. An output of the comparator circuit is connected to a control member which is adapted to control the power applied to the device by the power supply apparatus so that said variable is essentially equal to the desired value.
An example of such a power supply apparatus is described in Philips Technical Review 39 (1980), No. 2, pp. 37-47, notably with reference to FIG. 14. The known power supply apparatus is intended to power a semiconductor laser and includes, a photodiode which is accommodated in the same envelope as the laser for generating a photocurrent which is proportional to the light flux of the laser and which constitutes the test signal. The power applied to the laser in the known power supply apparatus can be controlled so that the current produced by the photodiode (monitor) remains constant at a desired value. The control of only one variable, however, involves the risk that the value of another laser variable is no longer within the desired range or, even worse, no longer within the safe range. Driving a semiconductor laser diode beyond the safe working range can readily damage the laser. For safe operation of a laser, therefore, it would be desirable to control the power applied to the laser so that more than one of the laser variables is maintained at or near a desired value. In addition to said monitor current, such variables are, for example, the laser current and the laser voltage and the radiant power of the laser. However, in practice this is a difficult problem because the various variables are interrelated in a rather complex manner.